Digitally assisted ultra-fast bandwidth calibration of a programmable analog filter

ABSTRACT

Systems and methods for improving seek times in a read channel performing a read operation on a disk drive are disclosed. The method includes extrapolating, based on initial filter settings corresponding to a first user-data region or a first servo-data region and tracked settings varying over time, a proportionality constant for the programmable analog filter, in response to determining a new target frequency for the programmable analog filter corresponding to a second user-data region or a second servo-data region, computing new filter settings for the programmable analog filter based on the extrapolated proportionality constant, and transmitting, to the programmable analog filter, the computed new filter settings corresponding to the new target frequency of the second user-data region or the second servo-data region.

CROSS REFERENCE TO RELATED APPLICATION

This disclosure claims the benefit of commonly-assigned U.S. ProvisionalPatent Application No. 62/875,419, filed Jul. 17, 2019, which is herebyincorporated by reference herein in its entirety.

FIELD OF USE

This disclosure generally relates to the calibration of programmableanalog filters that may be suitable for self-servo-write operations instorage devices such as disk drives. More particularly, this disclosurerelates to systems and methods for improving seek times in operation ofdisk drives.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of theinventors hereof, to the extent the work is described in this backgroundsection, as well as aspects of the description that may not otherwisequalify as prior art at the time of filing, are neither expressly norimpliedly admitted to be prior art against the subject matter of thepresent disclosure.

Analog filters are commonly used in front-end devices to shape andoptimize dynamic range of analog signals before its digitization. Atypical filter may consist of multiple stages of filtering and built inequalization. Often such filters are reconfigurable and adaptable interms of the shape of transfer function, pole-zero location,equalization type, and bandwidth to adapt to change of operation, datarate, usage and application.

Specifically, for disk drive applications, storage media may consist ofa continuum of ‘data sectors’ (where user data is written to and readfrom) which are interspersed with ‘servo sectors’ (used to identifyposition on magnetic disc). Servo sectors may be written with a tonalsignature of a certain frequency, while a data sector may be writtenwith highly overlapping pulses with a broadband content. Thus, thefilter's operating frequency continuously shifts between data and servowedges. In an embodiment, the filter's operating frequency also shiftsbetween different data sectors (during a read operation) and servo zones(during a write operation) of the Hard-Disk Drive.

To be able to dynamically and accurately change channel/filter bandwidthbetween data and servo, analog filters require closed-loop andcontinuous bandwidth calibration, which tracks voltage variations aswell as temperature supply variations. Typical solutions of calibratingan analog filter's bandwidth consist of ‘sensing’ analog-time-constantand building a control loop around it to fix it to a‘golden-time-reference’. For instance, in some conventional techniques,an analog filter's bandwidth (analog-time-constant) is fixed to a givenpercentage of PLL frequency (golden-time-reference).

However, such closed-loop calibration of filter bandwidth is typically avery slow process and can take hundreds of micro-seconds to converge. Incontrast, length of the servo gate in a read channel may only span 5-10μs. This extensive time for calibration means that the read channelcannot be set immediately and also needs redundant and independentcalibration of data and servo. This directly impacts the formatefficiency and results in missed servo or data wedges and necessitatesthat head to go over the disk-track multiple times.

Although the description above discusses the advantages of ultra-fastsettling analog filters in the context of a disk drive application, askilled artisan will understand that the methods and techniquesdisclosed herein are suitable for additional front-end devices as wellas for reception of wireless analog signals (e.g., WiFi, Cellular,etc.). For example, one application of ultra-fast settling programmableanalog filters is in Zone Servo which include a few servo zones acrossthe Hard-Disk Drive. The filter needs to be driven rapidly to go fromone servo zone to another without having to wait for the replica cell tosettle. Moreover, on the servo side, there are now new features such asconcentric servo which also need fast settling to reconfigure theprogrammable analog filter. Similarly, data is often varied from sectorto sector as well. Thus, there is a need for ultra-fast settlingprogrammable analog filters on the data side as well.

SUMMARY

According to implementations of the subject matter of this disclosure, adisk drive includes a disk having a disk surface, where the disk surfaceincludes a plurality of tracks arranged in an embedded servo formatincluding a plurality of user-data regions and a plurality of servo dataregions, each data region includes a plurality of data zones in each ofwhich user data is stored in a plurality of track segments at a datachannel frequency particular to that data zone, and each servo regionincludes a plurality of servo zones in each of which servo data arestored in a plurality of track segments at a servo channel frequencyparticular to that servo zone and different from the data channelfrequency. The disk device also includes a read channel for performing auser-data read operation for reading data from the disk surface. Theread channel includes a digital processor configured to extrapolate,based on initial filter settings corresponding to a first user-dataregion or a first servo-data region and tracked settings varying overtime, a proportionality constant for the programmable analog filter, inresponse to determining a new target frequency for the programmableanalog filter corresponding to a second user-data region or a secondservo-data region, compute new filter settings for the programmableanalog filter based on the extrapolated proportionality constant, andtransmit, to the programmable analog filter, the computed new filtersettings corresponding to the new target frequency of the seconduser-data region or the second servo-data region.

In a first implementation of such a method, the digital processor isconfigured to extrapolate the proportionality constant for theprogrammable analog filter by performing a background calibrationoperation to determine the initial filter settings for the programmableanalog filter corresponding to the first user-data region or the firstservo-data region, and determining a ratio between a transconductancevalue generated based on the determined initial filter settings andinitial target bandwidth corresponding to the first user-data region orthe first servo-data region, wherein the ratio is constant overtemperature and voltage variance.

In a first instance of that first implementation, estimating theproportionality constant based on the initial target bandwidth and thecorresponding normalized transconductance value includes determining anelement control word corresponding to the initial target bandwidth,computing an initial transconductance value corresponding to thedetermined element control word, and estimating the proportionalityconstant based on the computed initial transconductance value.

That first instance of that first implementation may include computingthe initial transconductance value by searching a weight-lookup table toidentify an initial fine-tuning control value and an initial coarsecontrol value corresponding to the determined element control word.

A second instance of the first implementation may further includedetermining new filter settings for the programmable analog filter basedon the extrapolated proportionality constant by computing a desiredbandwidth based on a new target phase-locked loop frequency and a newfrequency control word, computing a new transconductance value based onthe estimated proportionality constant and the computed desiredbandwidth, and determining the new filter settings for the programmableanalog filter based on the computed new transconductance value.

The second instance of the first implementation may further includecomputing the desired bandwidth in accordance with:Desired Bandwidth=FCW_new*PLL_Frequence_new

wherein FCW_new is the new frequency control word and PLL_Frequency_newis the new target phase-locked loop frequency.

The second instance of the first implementation may further includedetermining the fine-tuning control value and the coarse control valuefor the programmable analog filter based on the computed new normalizedtransconductance value by searching a weight-lookup table to identifythe new filter settings corresponding to the computed newtransconductance value, wherein the filter settings include afine-tuning control value and a coarse control value.

In an embodiment, the weight-lookup table is pre-computed based on anominal characteristic curve of a plurality of transconductor elementsand Digital-to-Analog Converter (DAC) codes.

In one instance of the first implementation, the fine-tuning controlvalue corresponds to a biasing voltage applied to the programmableanalog filter.

In yet another instance of the first implementation, the coarse controlvalue corresponds to a number of unit elements connected in parallelwithin the programmable analog filter.

According to implementations of the subject matter of this disclosure, amethod for improving seek times in a read channel performing a readoperation on a disk drive is disclosed. The method includesextrapolating, based on initial filter settings corresponding to a firstuser-data region or a first servo-data region and tracked settingsvarying over time, a proportionality constant for the programmableanalog filter, in response to determining a new target frequency for theprogrammable analog filter corresponding to a second user-data region ora second servo-data region, computing new filter settings for theprogrammable analog filter based on the extrapolated proportionalityconstant, and transmitting, to the programmable analog filter, thecomputed new filter settings corresponding to the new target frequencyof the second user-data region or the second servo-data region.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the disclosure, its nature and various advantages,will be apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout, and inwhich:

FIG. 1 shows an example of a system architecture for the improvedbandwidth calibration methods in accordance with subject matter of thepresent disclosure;

FIG. 2 is a diagram of an N-stage analog filter as in FIG. 1 inimplementations of the subject matter of this disclosure;

FIG. 3 is a diagram of a stage of the N-stage analog filter as in FIG. 2in accordance with implementations of the subject matter of thisdisclosure;

FIG. 4 is a diagram showing the graphical relationship between a DACcode and DAC Control Weight;

FIG. 5 is a diagram showing the graphical relationship between a dialedbandwidth and a normalized transconductance;

FIG. 6 is a transconductance mapping table in accordance withimplementations of the subject matter of this disclosure;

FIG. 7 is a flow diagram showing a method for improved seek times duringoperation of a disk drive in accordance with implementations of thesubject matter of this disclosure;

DETAILED DESCRIPTION

As discussed above, Hard-Disk Drives are often divided into multiplezones, where each of the zones has its own respective writingfrequencies. Modern hard drives can have around 15-20 zones thereforerequiring multiple calibration operations. Conventional systemsrequiring full calibration can take 80-100 μs to settle for each zone. Abandwidth calibration system and method in accordance with the presentdisclosure provides a fast-settling operation which removes latenciesassociated with waiting for full calibration for each of the zones.Specifically, methods and systems in accordance with the currentdisclosure can perform the filter's bandwidth calibration in as short atime as ˜1 μs.

Similarly, data is often varied when written into different sectors of aHard-Disk Drive. A read channel device needs to switch to new frequencytriggers as it moves from one sector to the next. Conventional systemsrequiring full calibration have a lot of latency associated with theslow settling operation of the analog filter. In contrast, acomputation-based bandwidth calibration system as discussed here in thecurrent disclosure provides a fast-settling programmable analog filterwhich reduces system delay and increases format efficiency.

As described above, conventional methods require waiting for a fullclosed-loop calibration of the filter's time constant that searches forthe filter's calibrated setting. Specifically, conventional calibrationof the filter's bandwidth involves an extensive search on the filter'scontrol setting for which the bandwidth of the filter accurately matchesa target bandwidth. Target bandwidth is defined through aFrequency-Control-Word (FCW) in proportion to a golden frequency or timereference, such as a locked Phase-Locked-Loop Frequency (PLL_Frequency).For example, the target frequency is defined as:Target_(Frequency)=FCW*PLL_(Frequency)  (1)

where Frequency-Control-Word (FCW) and PLL_frequency could varysignificantly between data and servo.

A typical closed-loop bandwidth calibration relies on a mastercalibration circuit which is a replica of the main filter in terms oftime-constant/bandwidth. Calibration logic circuit searches through thefilter controls to find the closest match between the time constant ofthe replica circuit to the inverse of the target bandwidth as defined byEquation (1) above. Convergence of this traditional search is impactedby slow settling of analog components and the vast span of controlsettings to comb through. Thus, the traditional bandwidth calibrationcould take 10 μs to 100 μs to converge on a solution. Both the filterand the read channel have to wait before the controls can be updated toreflect the desired bandwidth, thus impacting format efficiency.

In contrast, in accordance with an implementation of the subject matterof this disclosure, a digital computational engine extrapolates anddirectly finds the new settings for the filter based on aproportionality constant for the analog filter that has been previouslyderived from a background closed-loop calibration. More specifically, amaster replica in background computes a proportionality constant betweenfilter's bandwidth and its control setting. This proportionalityconstant is computed prior to the time when a new frequency target isdesired (e.g., receiving a new frequency target trigger when shiftingbetween data and servo wedges in a disk drive application). Furthermore,the filter's control settings are defined through a weight consisting ofappropriate summation of analog components. A map of weights that isindexed by a set of discrete analog controls (coarse and fine) is usedfor lookup purposes to define weight ratios between the analogcomponents. When a newly desired frequency bandwidth is dialed, a priorestimate of the proportionality coefficient along with the preloaded mapof filter weights is used to compute the desired filter weight whichcorresponds to the desired filter settings. This weight directly yieldsthe computed solution for the filter's calibration as will be describedbelow in greater detail.

The subject matter of this disclosure may be better understood byreference to FIGS. 1-7.

A disk drive includes a disk having a disk surface. The disk surfaceincludes a plurality of tracks arranged in an embedded servo formatincluding a plurality of user-data regions and a plurality of servo dataregions. In an embodiment, each data region includes a plurality of datazones in each of which user data is stored in a plurality of tracksegments at a data channel frequency particular to that data zone.Similarly, each servo region includes a plurality of servo zones in eachof which servo data are stored in a plurality of track segments at aservo channel frequency particular to that servo zone and different fromthe data channel frequency. Moreover, the disk drive includes a readchannel for performing a user-data read operation for reading data fromthe disk surface. The structure and operation of the read channel isdescribed in greater detail below in connection with FIG. 1.

FIG. 1 shows an example system architecture for the improved bandwidthcalibration methods in accordance with subject matter of the presentdisclosure. The system includes a programmable analog filter 102 havingan input 103 and an output 104, a replica cell 105, and a digital statemachine 106. Analog filter 102 (as shown in FIG. 2 in greater detail) isa multi-stage filter designed to provide 5th order filtering andfrequency selective shaping. The filtering function is achieved bycombining a lossy element (e.g., transconductor or resistors) withcapacitive impedances to yield appropriate shaping over a frequencyband, in accordance with an embodiment of this disclosure.

Tuning of the filter over a wide bandwidth is achieved by controllingelements 204-1, 204-2, 204-3, 204-4 (collectively 204) of each stage202-1, 202-2, 202-3, 202-4, 202-5 (collectively 202) of the filter via acombination of coarse control and fine control. Coarse control (e.g.,GM-codes [0:3]) controls number or multiplicity of unit filteringelements to be put in parallel (for example, MOS-resistors,transconductors, and capacitors). Fine control controls the bias voltageassociated with the filter elements (bias of the transconductors or theMOS-resistor could be controlled through a Current Mode DAC 302).

As discussed above, appropriate selection of coarse and fine control forfilter elements set the frequency of analog filter 102 close to thedesired frequency. Each unique combination of control elements (coarseand fine) is expressed as Element Control Word (ECW). The requiredElement Control Word (ECW) to set the bandwidth of the analog filtervaries from part to part, over process, and even drifts over time andtemperature. Hence, conventional methods require a closed-loopcalibration engine to appropriately find and then track this value.

Digital state machine 106 contains operational and logical functionsthat control the replica cell 105 that runs in background, intakesinformation regarding newly desired filter bandwidth, computes thetransconductance weight that would be required to achieve the desiredfilter bandwidth, indexes this computed transconductance weight withregards to filter elements, and finally applies an ECW (filter control)to analog filter 102 to appropriately set its frequency. Morespecifically, following the first frequency trigger after power-on, thebackground calibration engine finds an appropriate Element Control Word(ECW) (coarse and fine control of filter) corresponding to the initialbandwidth target. Next, the digital state machine 106 computes aproportionality constant based on the initial bandwidth target and theECW corresponding to the initial bandwidth target. Once a new frequencytarget is dialed (e.g., the dialed data or servo mode PLL frequency),digital state machine 106 computes the newly desired filter bandwidthusing Equation (4) discussed below. Next, the digital state machine 106determines the ECW corresponding to the newly desired bandwidth targetbased on the proportionality constant and the newly desired filterbandwidth. Additional details about the operation of the digital statemachine 106 is discussed below in greater detail in connection with FIG.7. This entire computation and estimation process takes approximately 1μs.

A background replica-GM based master calibration loop runs in thebackground using replica cell 105. This calibration loop estimates theanalog filter's proportionality constant and compares it to thereference set through a golden time reference (PLL_frequency) and theFCW (Frequency Control Word). Subsequently, the ECW (Element ControlWord) is varied to achieve the state where the filter's bandwidth issame as the dialed bandwidth set by the FCW. This background calibrationloop is slow, runs in background, can run at an arbitrary bandwidthtarget, and does not need to be synced to the main path filter where acertain bandwidth target is desired.

Purpose of this background calibration loop is to establish theproportionality between a certain bandwidth and Element control word andhave this proportionality constant estimated while tracking its driftsover temperature and time. More specifically, once the backgroundcalibration loop finds an appropriate Element Control Word (ECW) (coarseand fine control of filter) corresponding to the initial bandwidthtarget after power-on, the effective weight of analog components(gm_eff) as indexed by filter's discrete control ECW (both coarse andfine control) is mapped against the target bandwidth and a slope of theresulting curve is calculated. This computed slope is theproportionality constant which establishes the proportionality between acertain bandwidth and Element control word.

As illustrated in FIG. 5, the initial target bandwidth 502 is mappedagainst the computed effective weight of analog components (gm_eff) 504and a slope 510 is calculated. As will be discussed in the greaterdetail below, once a new target bandwidth 506 is dialed, the calculatedslope 510 is used to extrapolate a corresponding new effective weight ofanalog components (gm_eff) 508. Once the new effective weight of analogcomponents (gm_eff) 508 is determined, digital state machine 106performs a lookup operation using the GM-weight mapping table 108.

More specifically, digital state machine 106 is associated with aGM-weight mapping table 108. As shown in FIG. 6, GM-weight mapping table108 follows many-to-one mapping from its indices (coarse and finecontrol) to its function values (gm_eff). That is, multiple combinationsof coarse control (GM Code) and fine control (DAC Code) yields to thesame gm_eff values. As discussed above, an Element Control Word (ECW)which defines a unique combination of analog filter elements can beassociated to a nominal analog weight which is proportional to filter'sbandwidth.

In an example embodiment, the programmable analog filter is atransconductance-C filter. Transconductance (Gm) is an expression of theperformance of a bipolar transistor or field-effect transistor. A Gm-Canalog filter's (such as the programmable analog filter 102) bandwidthis proportional to its effective equivalent transconductance (gm_eff)but inversely proportional to the integrating capacitor—which can berepresented by Equation (2) below:Filter's Bandwidth∝gm_eff/C  (2)

In terms of filter's elements, transconductance (gm_eff) can beexpressed as a combination of discrete weights (plurality of unitelements) and fine-weights (e.g., Current Mode DAC 302 providing finecontrol by managing the bias current applied to the programmable analogfilter 102). For example, as shown in FIG. 3, multiple of transconductorelements (GM) of varying fingered transistors are connected in parallel,forming a coarse control weight corresponding to total number offingers. An example GM-control mapping is shown in the table (1) below:

GM-Control-Weight = GM-Code [3:0] Total No. of Fingers 0001 1 0011 30111 6 1111 11

Furthermore, bias current of the entire GM cell is varied with a fineresolution DAC (e.g., Current Mode DAC 302), where DAC control weightcaptures the relationship between nominal transconductance andDAC-codes.Transconductance−weight=gm_eff=(GM control−weight)*(DAC controlweight)  (3)

FIG. 4 shows a graphical relationship between a DAC code and DAC ControlWeight. Based on Equation (3), a map of the filter's control isgenerated that has one-to-one correspondence between individual controlsof ECW (GM control, DAC control, etc.) to a weight associated with thattransconductance. An example of gm_eff mapping table is illustrated inFIG. 6 This weight table is pre-computed based on plurality of GMelement control and DAC'S nominal characteristic curve using Equation(3) above.

Typically, gm_eff-DAC characteristic curve (as shown in FIG. 4) wouldhave variations over temperature that are not captured in the mappingtable. However, in accordance with embodiments disclosed herein, onlyratios of the respective weights are critical for the accuracy ofbandwidth calibration. Absolute weights (that vary widely withtemperature) are not significant as long as ratio-map of the weightremains stable across temperatures. Since a current-mode DAC is used forthe control of GM cells where ratio of bias currents is deterministicacross DAC code, and also since transconductors are in fairly linearregion of operation, the normalized ratio of DAC control weight does notvary significantly with process and temperature when measured across DACcodes.

Returning to FIG. 1, the system architecture further includes an analogcontrol multiplexer (MUX) 110 which multiplexes the ECW controls betweenthe ultra-fast computational engine and the full calibration using thereplica cell 105. In accordance with an embodiment, the MUX 110 alsoaligns updates to the analog filter with its operational region (data orservo). That is, the programmable analog filter is updated dynamicallywith corresponding control word as the filter transitions between dataand servo (or between different sectors during a read operation ordifferent servo zones during a write operation).

Embodiments in accordance with the above architecture detail anultra-fast bandwidth calibration scheme for analog filters. This systemarchitecture minimizes the latency between dialing a new frequency(servo or data trigger) and setting the read channel to appropriatebandwidth. System application of this scheme enables fast zone-servo aswell as ultra-fast readout for data for disk drive technologies.

As shown in FIG. 1, the background master calibration loop consists ofthe replica cell 105 whose time constant needs to be estimated and setto near the analog filter 102's bandwidth. The digital state machine 106consists of a weight-mapping table 108 which resides either in hardwareor firmware, in accordance with various embodiments of this disclosure.The mapping table 108 consists of effective weight of analog components(gm_eff) as indexed by filter's discrete control ECW (both coarse andfine control). Digital state machine 106 also includes a controller thatperforms pre-defined sequential operations/algorithm to fast compute thebandwidth calibration solution following the frequency trigger.

The operation of the system architecture of FIG. 1 is now described withmore particularity in combination with the flow diagram of FIG. 7. At702, a background calibration operation is performed to determineinitial filter settings for a programmable analog filter.

Specifically, following the first frequency trigger after power-on, thebackground calibration engine (using replica cell 105) begins operation.This background calibration finds an appropriate Element Control Word(ECW) (coarse and fine control of filter) corresponding to the bandwidthtarget. Bandwidth target is expressed as multiplication of FrequencyControl Word (FCW) and the dialed data or servo mode PLL frequency.Bandwidth Target=FCW*PLL_Frequency_Word  (4)

As background calibration ends, the flow diagram proceeds to 704, wherea proportionality constant for the programmable analog filter isextrapolated based on the determined initial filter settings.Specifically, Element Control Word corresponding to the calibratedfilter along with the filter's weight control table is used to compute anormalized gm_eff that corresponded to that calibration solution. Inaccordance with an embodiment, this process is a lookup operationcarried out on the hardware table that is indexed through ECW obtainedat 702.

Once the normalized gm_(eff) is computed for a given bandwidth target,the proportionality constant for the filter is computed using thefollowing equations:Filter bandwidth∝gm_eff/Capacitor_weight  (5)Filter bandwidth=K_eff*gm_eff/Capacitor_weight  (6)Filtering Time Constant (K_eff)=Filterbandwidth*Capacitor_weight/gm_eff   (7)

This proportionality constant K_eff depends on analog characteristics ofthe filter and varies over parts and can even drift over temperatureand/or voltage. In accordance with an embodiment of the currentdisclosure, estimation of this proportionality constant K_eff through abackground calibration using the replica cell 105 tracks these driftsand variations.

Steps 702 and 704 are performed prior to receiving a newly dialed targetfrequency (e.g., receiving a new frequency target trigger when shiftingbetween data and servo wedges in a disk drive application). Inaccordance with an embodiment, once the analog filter's proportionalityconstant has been determined after the initial power-on, steps 702 and704 are performed in parallel. Once a new frequency trigger is received,the process proceeds to 706, where new filter settings for theprogrammable analog filter are determined based on the extrapolatedproportionality constant. Specifically, first, the new target bandwidthfor the analog filter 102 is computed based on the newly dialed PLLfrequency and the newly dialed frequency control word:New Bandwidth Target=FCW_new*PLL_Frequency_new  (8)

Based on the previously determined filter's proportionality constant(K_eff at 704), newly dialed filter's bandwidth target (from Equation 8)and newly adjusted Capacitance weight (firmware dialed), the new gm_effneeded to achieve the bandwidth target is computed based on Equation 6above.

Once the computed new gm_eff is available, the digital state machine 106accesses the GM-weight table 108 to determine the filter's coarse andfine tuning control (FCW) by choosing values from the weight matrixwhich are closest to the computed new gm_eff. More specifically, digitalstate machine 106 accesses the GM-weight table 108 to determine theappropriate GM code (coarse control) and the DAC code (fine control)that correspond to the computed new gm_eff.

As shown in FIG. 6, the filter 102's Gm-weight table 108 followsmany-to-one mapping from its indices (coarse and fine control) to itsfunction values (gm_eff). That is, multiple combinations of coarse andfine control yields to the same gm_eff values. In one such embodiment, arule-based lookup process is used to index FCW. For example, a higherfine control is preferred if two set of controls map to same gm_eff, inaccordance with one embodiment of the current disclosure.

Moreover, the new gm_eff computed above is normalized to the filter'sweight table. Hence, these are based on a correspondence and equivalencerelation established between Filter's control Word (FCW) and thefilter's bandwidth (established through background calibration) alongwith pre-determined weight-ratio table. In reality, transconductance ofthe filter components vary widely with temperature and/or voltagedrifts. However, this variation impacts all the weights proportionallywhile keeping the ratio of weights intransient. Because this commonvariation is captured as a proportionality constant which is estimatedin background (through replica cell 105) and is updated periodically,the absolute weight of the gm_eff mapping table does not matter.

In accordance with an embodiment, searching and indexing of the gm_efflookup table 108 is implemented as rule-based search or a parallelcomparison of a target gm_eff to the cells of the table. With each ofthese implementations, step 706 is completed within a few cycles ofavailable clock (10¬40 cycles of reference clock). In the currentimplementation, fast solution to filter's bandwidth calibration istherefore available in as short a time as ˜1 μs.

Once a fast solution to the filter's bandwidth calibration is available,the process proceeds to step 708 and the filter settings are immediatelyapplied to the channel as the channel switches between Data/Servo. Moreparticularly, the appropriate GM code (coarse control) and the DAC code(fine control) that correspond to the computed new gm_eff aretransmitted to the programmable analog filter 102. For example, based onthe transmitted GM code, a number of transconductor elements ofvarying-fingered transistors could be connected in parallel, thusforming a coarse control weight corresponding to the total number offingers. Similarly, based on the transmitted DAC code, bias current ofthe entire transconductor cell is varied using the Current Mode DAC 302.

It is noted that the foregoing is only illustrative of the principles ofthe invention, and that the invention can be practiced by other than thedescribed embodiments, which are presented for purposes of illustrationand not of limitation, and the present invention is limited only by theclaims which follow.

What is claimed is:
 1. A disk drive comprising: a disk having a disksurface, wherein: the disk surface includes a plurality of tracksarranged in an embedded servo format including a plurality of user-dataregions and a plurality of servo data regions; each data region includesa plurality of data zones in each of which user data is stored in aplurality of track segments at a data channel frequency particular tothat data zone; and each servo region includes a plurality of servozones in each of which servo data are stored in a plurality of tracksegments at a servo channel frequency particular to that servo zone anddifferent from the data channel frequency; a read channel for performinga user-data read operation for reading data from the disk surface,wherein the read channel comprises: a digital processor configured to:extrapolate, based on initial filter settings corresponding to a firstuser-data region or a first servo-data region and tracked settingsvarying over time, a proportionality constant for a programmable analogfilter; in response to determining a new target frequency for theprogrammable analog filter corresponding to a second user-data region ora second servo-data region, compute new filter settings for theprogrammable analog filter based on the extrapolated proportionalityconstant; and transmit, to the programmable analog filter, the computednew filter settings corresponding to the new target frequency of thesecond user-data region or the second servo-data region.
 2. The diskdrive of claim 1, wherein the digital processor is configured toextrapolate the proportionality constant for the programmable analogfilter by: performing a background calibration operation to determinethe initial filter settings for the programmable analog filtercorresponding to the first user-data region or the first servo-dataregion; and determining a ratio between a transconductance valuegenerated based on the determined initial filter settings and initialtarget bandwidth corresponding to the first user-data region or thefirst servo-data region, wherein the ratio is constant over temperatureand voltage variance.
 3. The disk drive of claim 2, wherein the digitalprocessor is configured to extrapolate the proportionality constant forthe programmable analog filter by: determining an element control wordcorresponding to the initial target bandwidth corresponding to the firstuser-data region or the first servo-data region; computing an initialtransconductance value corresponding to the determined element controlword; and estimating the proportionality constant based on the computedinitial transconductance value.
 4. The disk drive of claim 3, whereinthe digital processor is configured to compute the initialtransconductance value by: searching a weight-lookup table to identifyan initial fine-tuning control value and an initial coarse control valuecorresponding to the determined element control word.
 5. The disk driveof claim 1, wherein the digital processor is configured to compute newfilter settings for the programmable analog filter based on theextrapolated proportionality constant by: computing a desired bandwidthbased on a new target phase-locked loop frequency and a new frequencycontrol word corresponding to the second user-data region or the secondservo-data region; computing a new normalized transconductance valuebased on the computed proportionality constant and the computed desiredbandwidth; and determining the new filter settings for the programmableanalog filter based on the computed new transconductance value.
 6. Thedisk drive of claim 5, wherein the desired bandwidth is computed inaccordance with:Desired Bandwidth=FCW_new*PLL_Frequency_new; wherein FCW_new is the newfrequency control word and PLL_Frequency_new is the new targetphase-locked loop frequency.
 7. The disk drive of claim 5, wherein thedigital processor is configured to determine the new filter settings forthe programmable analog filter by: searching a weight-lookup table toidentify the new filter settings corresponding to the computed newtransconductance value, wherein the filter settings include afine-tuning control value and a coarse control value.
 8. The disk driveof claim 7, wherein the weight-lookup table is pre-computed based on anominal characteristic curve of a plurality of transconductor elementsand Digital-to-Analog Converter (DAC) codes.
 9. The disk drive of claim7, wherein the fine-tuning control value corresponds to a biasingvoltage applied to the programmable analog filter.
 10. The disk drive ofclaim 7, wherein the coarse control value corresponds to a number ofunit elements connected in parallel within the programmable analogfilter.
 11. A method for improving seek times in a read channelperforming a read operation on a disk drive, the disk drive comprising adisk having a disk surface, wherein: the disk surface includes aplurality of tracks arranged in an embedded servo format including aplurality of user-data regions and a plurality of servo data regions;each data region includes a plurality of data zones in each of whichuser data is stored in a plurality of track segments at a data channelfrequency particular to that data zone; and each servo region includes aplurality of servo zones in each of which servo data are stored in aplurality of track segments at a servo channel frequency particular tothat servo zone and different from the data channel frequency; themethod comprising: extrapolating, based on initial filter settingscorresponding to a first user-data region or a first servo-data regionand tracked settings varying over time, a proportionality constant for aprogrammable analog filter; in response to determining a new targetfrequency for the programmable analog filter corresponding to a seconduser-data region or a second servo-data region, computing new filtersettings for the programmable analog filter based on the extrapolatedproportionality constant; and transmitting, to the programmable analogfilter, the computed new filter settings corresponding to the new targetfrequency of the second user-data region or the second servo-dataregion.
 12. The method of claim 11, wherein the proportionality constantfor the programmable analog filter is extrapolated by: performing abackground calibration operation to determine the initial filtersettings for the programmable analog filter corresponding to the firstuser-data region or the first servo-data region; and determining a ratiobetween a transconductance value generated based on the determinedinitial filter settings and initial target bandwidth corresponding tothe first user-data region or the first servo-data region, wherein theratio is constant over temperature and voltage variance.
 13. The methodof claim 12, wherein the proportionality constant for the programmableanalog filter is extrapolated by: determining an element control wordcorresponding to the initial target bandwidth corresponding to the firstuser-data region or the first servo-data region; computing an initialtransconductance value corresponding to the determined element controlword; and estimating the proportionality constant based on the computedinitial transconductance value.
 14. The method of claim 13, wherein theinitial transconductance value is computed by: searching a weight-lookuptable to identify an initial fine-tuning control value and an initialcoarse control value corresponding to the determined element controlword.
 15. The method of claim 11, wherein the new filter settings forthe programmable analog filter is computed based on the extrapolatedproportionality constant by: computing a desired bandwidth based on anew target phase-locked loop frequency and a new frequency control wordcorresponding to the second user-data region or the second servo-dataregion; computing a new normalized transconductance value based on thecomputed proportionality constant and the computed desired bandwidth;and determining the new filter settings for the programmable analogfilter based on the computed new transconductance value.
 16. The methodof claim 15, wherein the desired bandwidth is computed in accordancewith:Desired Bandwidth=FCW_new*PLL_Frequency_new; wherein FCW_new is the newfrequency control word and PLL_Frequency_new is the new targetphase-locked loop frequency.
 17. The method of claim 15, wherein the newfilter settings for the programmable analog filter is determined by:searching a weight-lookup table to identify the new filter settingscorresponding to the computed new transconductance value, wherein thefilter settings include a fine-tuning control value and a coarse controlvalue.
 18. The method of claim 17, wherein the weight-lookup table ispre-computed based on a nominal characteristic curve of a plurality oftransconductor elements and Digital-to-Analog Converter (DAC) codes. 19.The method of claim 17, wherein the fine-tuning control valuecorresponds to a biasing voltage applied to the programmable analogfilter.
 20. The method of claim 17, wherein the coarse control valuecorresponds to a number of unit elements connected in parallel withinthe programmable analog filter.